1. Field of The Invention
The present invention relates to a method for re-utilizing steels, especially scrap steels, and more particularly, to a method for making a reclaimed steel by providing scrap steels formed thereon with a Sn surface layer mainly composed of Sn, removing the Sn surface layer from the scrap steels, and then refining the scrap steels by melting.
2. Description of the Background Art
Sn-plated steels are formed with a stable oxide film on the surface thereof, and thus, have a beautiful gloss. These steels are widely used as containers for foods and as cans for beverage products. Recently, much progress has been made in recycling steel beverage containers from the standpoint of effective utilization of resources. In this manner, steels can be re-used. For the re-utilization, i.e., recycling, of scrap steels, the usual practice is to initially charge a scrap steel into a melting furnace and then melt the steel, where a flux is added in order to collect and remove impurity elements as a slag. However, Sn is an element which is difficult to remove under the steel making conditions currently used, and is thus liable to remain in the molten steel. If Sn is present in the steel, not only is the quality of the steel degraded, but also Sn segregates in the grain boundary at the time of heating for rolling during the course of steel making, so that the hot workability of the steel is considerably reduced. In fact, if the amount of Sn in the molten steel is high enough, then it may be impossible to obtain a reclaimed steel having the desired physical properties. Accordingly, where Sn is present in molten steel in amounts exceeding its tolerance limit, it is usual to dilute the molten steel with a molten steel having a reduced amount of Sn in order to obtain a steel having the tolerance limit of Sn.
Many methods have been hitherto proposed for removing a Sn surface layer from Sn-coated steels and these methods include, in addition to the above-mentioned method using fluxes, a method wherein Sn is removed from a molten steel by evacuating a melting furnace under vacuum, thereby permitting low melting Sn to be converted to and evaporated as a gaseous component. However, leaving molten steel under vacuum leads to poor production efficiency and is, therefore, generally not suitable for industrial operations.
Several attempts have been made to remove the Sn surface layer prior to the melting of scrap steels, including (1) an alkali electrolytic method wherein a scrap steel is immersed in an alkali aqueous solution, to which a potential is applied thereby causing Fe to be passivated and Sn to be acceleratedly dissolved, and (2) a sulfurization method wherein an Sn-plated steel sheet is sulfurized in the presence of sulfur thereby causing Sn to be separated as SnS. The former alkali electrolytic method (1) is poor in efficiency, with the attendant problem that the production cost is too high. The latter sulfurization method (2) suffer from the problem of how to treat the sulfur-containing waste gases, and this method has not been put into practice.
In order to provide a method for efficiently removing an Sn surface layer in a solid state without involving any treatment of waste gases and incurring high costs, we have already proposed, in Japanese Laid-open Patent Application Hei 7-145431, a method wherein Sn-plated scrap steels are heated in an oxidative atmosphere at 500 to 1000.degree. C. to convert the Sn surface layer to corresponding oxides, followed by mechanical separation of the oxides.
This method requires a large amount of heat energy. In this process, and a large amount of a hot exhaust gas is produced in the melting furnace. This exhaust has is discharged from a melting furnace, to the outside environment. Therefore, the thermal energy contained in this exhaust gas is wasted. Accordingly, this process may be improved upon by effectively utilizing the heat energy of the exhaust gas. In other words, in order to further improve the production efficiency of this method, it is important that the heat loss is reduced to an extent as small as possible by combining the Sn-removing step and the melting step. In addition, the removal rate of Sn to be removed by the method is as low as about 40 to 50%. Accordingly, there is a demand of improving the Sn removal rate.